1. Field of the Invention
The inventive concept relates to a semiconductor memory device, and more particularly, to a device to improve data restoration capability in a volatile semiconductor device such as a dynamic random access memory (DRAM).
2. Description of the Related Art
Semiconductor memory devices include volatile semiconductor memory devices and non-volatile semiconductor memory devices. The volatile semiconductor memory device exhibits fast read and write speeds but the stored contents are lost when an external power supply is discontinued. That is, for a memory cell for storing data in the volatile semiconductor memory device, the stored data is not maintained over a predetermined time because of its intrinsic leakage current. To compensate for the limit, a system regularly performs a refresh operation so that a memory device may restore data.
The refresh operation is a series of operations of amplifying the data stored in the memory cell by using a sense amplifier and then storing the data in the memory cell again. The refresh operation includes an auto refresh mode in which the refresh operation is performed during a normal operation and a self refresh mode in which the refresh operation is performed when the system does not operate for a long time, for example, in a ready mode or a low power consumption mode. In the self refresh mode, the refresh operation is continuously performed at a certain interval without any external command.
In general DRAMs, the refresh operation of a memory device is constantly performed according to a refresh signal having a predetermined cycle. Thus, when the refresh cycle is slow, data may be lost. When the refresh cycle is excessively fast, the consumption of power increases.
When, during a data write operation, a sufficient amount of charges is not stored in the memory cell due to insufficient data write time or insufficient voltage, the memory cell may become a weak cell that is highly likely to lose data. When the weak cell exists and further the refresh cycle is slow, data loss possibility may increase accordingly.